In a mobile telecommunication system, such as a CDMA (Code Division Multiple Access) or WCDMA (Wide-band CDMA) or TDMA (Time division Multiple Access) system, transmission power levels between a base station (BS) and a mobile station (MS) associated with said BS can be continuously adjusted during an ongoing connection between the BS and the MS. This is done in order to provide a sufficient quality for the required transmission power levels as low as possible at the same time. By means of this it is possible to avoid “wasting” any network resources and MS battery resources, and to enable as great a number of mobile stations as possible to communicate simultaneously with the same BS having only limited power resources.
One system of power control is based on Power Control (PC) commands transmitted from one station to another to cause the other station to alter its power. The commands can be transmitted e.g. in a WCDMA closed loop. The closed loop power control mechanism between the BS and MS is used for equalising the power of signals from the MS at the BS input and also for compensating fast power deviations from the nominal level. These closed loop PC (CL PC) commands can be sent both in the uplink (towards the base station) and in the downlink (towards the mobile station), whereafter the BS or the MS will process the received command and reduce/increase its transmission power towards the receiving station (i.e. MS or BS respectively) accordingly.
For example, in the currently proposed WCDMA system it is envisaged that an outer loop PC generated by a radio network controller (RNC) of the WCDMA system will attempt to set the connection quality target (that the closed loop follows) of a physical connection between the BS and MS to be such that the required FER (Frame Error Ratio) target of the connection is met with a minimal connection quality target. The connection quality target can be announced e.g. by means of a so called Eb/No (Signalling Energy/Noise) target or SIR (signal to Interference Ratio) target or a similar parameter indicating some quality measurement for the connection. The relationship is such that the connection quality target (e.g. the SIR target) has to be set such that the FER remains at an appropriate level. The actual connection quality value (e.g. SIR) is then adjusted in accordance with the target value, and should follow any changes in the target value. The idea behind this is that by increasing the connection quality target value the connection quality will increase and the FER will improve.
However, if the FER target cannot be met due to e.g. a limitation in the available transmission power when severe interference or attenuation is predicted, the connection quality target will start increasing even though this rise in the connection quality target will not help in causing a better connection between the MS and the BS. If the power limitation is caused by a temporary lack of power caused by a condition such as slow fading or a temporarily weak connection (if, for instance, the MS is situated temporarily in a tunnel or cellar), the quality target will be unnecessarily high once this condition has been removed. This will result in an excessively high transmitted power until the quality target has returned to its normal (appropriate) level. At the BS side this unnecessarily used power resource could be used for transmission towards other mobile stations. At the MS side this will lead among other things, to unnecessary high power consumption and to a possible disturbance to other radio and/or electronic devices.
To give a more precise example, if the BS runs out of power (i.e. a power limitation situation occurs), then the mobile station MS will experience a higher FER than the set FER target. This will result (if not limited by some means) in an unlimited rise of the SIR target value. In accordance with one exemplifying possibility the average rate per frame of this rise can be given by the formula.rise_per_frame=(FER−FERth) step_size                where        FER is the actual obtained FER,        FERth is the FER target and        step_size is the step size of the outer loop algorithm        
Thus, if the actual FER is 2%, the FER target is 1% and the step size is 0.5 dB the SIR target will in ten seconds (1000 frames) be raised by 1%*1000*0.5 dB=5 dB, which can be considered to be a substantial rise. If the higher FER has been caused by e.g. shadowing and the situation changes suddenly the SIR target will be much too high for a while after this condition ends. In this specific example, the SIR target would decrease gradually back to its appropriate value in approximately 5/0.0005=1000 frames=10 seconds.
Earlier proposals to solve this problem have been based on setting absolute limits on the values of the quality targets. There are, however, some problems associated with this type of solution. Firstly, the set absolute limits have to be relatively loose due to the variations in the required quality target for satisfactory quality of the communication. Secondly, the setting of absolute limits for the MS is problematic due to the fact that the absolute value of the quality value setpoint depends heavily on the used estimation method.